Device for preparing thermoplastic material

ABSTRACT

An apparatus for reprocessing thermoplastic synthetic plastic material has a receptacle (1) for accomodating the synthetic plastic material, in which receptacle (1) has a rotating tool (4) is provided which presses the synthetic plastic material out of the receptacle (1) into the inlet opening (11) in the side wall of the screw housing (12) of a screw (13). The screw housing (12) of the screw (13) has a pocket-like enlargement (22) in the area of the inlet opening (11), which enlargement (22) provides an additional space for the synthetic plastic material being pressed in. On that edge of the inlet opening (11) on which the direction of rotation of the screw (13) is directed towards the receptacle (1), this enlargement (22) is confined by an adjustable wall piece (23). The enlargement (22) gives way in an area of the screw housing (12) adjoining it to grooves (28), which are of a depth which decreases to the screw diameter and which are separated from each by the rigid ribs (29) projecting toward the screw (13) from the wall of the enlargement (22). Each of these grooves (28) forms a space for the synthetic plastic material, the longitudinal direction of which space runs essentially in the axial direction of the screw (13).

BACKGROUND OF THE INVENTION

The invention refers to an apparatus for reprocessing thermoplasticsynthetic plastic material, comprising a receptacle for the syntheticplastic material to be processed, which receptacle has a device with atleast one rotating tool which conveys the synthetic plastic materialthrough a discharge opening of the receptacle such that a component ofstuffing force is exerted on the synthetic plastic material in the axialdirection of this discharge opening and transmitted thereto always bythe synthetic plastic material subsequently being pressed in, whichdischarge opening leads to an inlet opening for the synthetic plasticmaterial arranged in the side wall of the screw housing, whereby theinterior surface of the housing of the screw in the area of the inletopening has a pocket-like enlargement which provides an additional spaceto accomodate synthetic plastic material and which is bound by anadjustable wall piece on that edge of the inlet opening on which thedirection of rotation of the screw is directed towards the receptacle,which wall piece allows the enlargement to be at least partially closedoff from the receptacle.

A device of this kind is the subject matter of an older, not previouslypublished construction by the present applicants. This construction hasthe advantage that even synthetic plastic materials possessing differentproperties can be reliably engaged by the screw since the stuffingaction of the rotating tool continuously ensures that sufficientsynthetic plastic material is pressed from the side between theindividual flights of the screw rotating around its longitudinal axis.Although this in itself poses the risk that the conveying efficiency ofthe screw becomes too great and causes overloading of the screw drive,this risk can be avoided by drawing back the adjustable wall piece inrelation to the screw such that part of the synthetic plastic materialengaged by the screw is returned to the receptacle. In this way it ispossible to adjust the conveying efficiency of the screw to theprevailing conditions.

SUMMARY OF THE INVENTION

The object of this invention is to further improve such an apparatus,particularly with regard to the feeding of loose materials into thescrew housing and with regard to manufacturing costs. The aforementionedolder construction, namely, has other wall pieces in the area of theinlet opening of the screw housing aside from the one mentioned above,some of which can be adjustable if required, and experience has shownthat the area of the screw housing with the inlet opening is expensiveto manufacture since its manufacture requires metal removal. Theinvention now improves the aforementioned apparatus with regard to thefeeding of loose materials into the screw housing and with regard to themanufacturing costs by having the enlargement give way to severalgrooves at least in the area of the screw housing adjoining it in thetransport direction of the screw, which grooves are of a depth whichdecreases to the screw diameter in this area and which grooves areseparated from each other in this area by rigid ribs projecting towardthe screw relative to the wall of the screw housing enlargement, each ofthese grooves providing a space for the synthetic plastic material, thelongitudinal direction of which space having a substantial componentextending in the axial direction of the screw housing.

The ribs confining the individual grooves create forces of resistance inthe area of the inlet opening of the screw housing for the syntheticplastic material engaged by the screw, which forces prevent the saidsynthetic plastic material from merely rotating along with the screwwithout being transported further to any substantial degree in the axialdirection of the screw. Owing to the fact that the ribs of the olderconstruction mentioned initially now no longer all have to be placed inthe area of the inlet opening for the synthetic plastic material, themachining of the screw housing section with the inlet opening issimplified. Although in extreme cases the ribs bounding the grooves mayproject into the area of the inlet opening, they do not extend along itsentire length as measured in the axial direction of the screw.Therefore, even in such a case, the grooves can be made on machines withonly a limited machining range.

To provide grooves placed in the area of the inlet opening of a screwhousing and in an area of it adjoining the said area in the axialdirection of the screw is known (DE-A 2 351 328). However, there, thegrooves form further recesses of the enlargement of the screw housing sothat the ribs remaining between the grooves do not project from the wallof the enlargement toward the screw. An arrangement of this kind wouldtherefore work less than ideally in the apparatus in accordance with theinvention.

In the scope of the invention, a preferred embodiment is to have theribs extend in the radial direction of the screw housing to theperiphery of the screw flights. Thereby it is most effectively preventedthat synthetic plastic material stuffed into the inlet opening andengaged by the screw merely rotates along with the screw without beingtransported in the direction of the axis.

Preferably, in accordance with a further variant of the invention, thegrooves are provided for only in one area of the screw housing adjoiningthe inlet opening and therefore do not extend into the area of the inletopening. This substantially simplifies the manufacture of the latterarea and makes it possible, in accordance with a preferred embodiment ofthe invention, to have the grooves in a sleeve forming a separatesection of the screw housing, which sleeve is installed into the rest ofthe screw housing such that it is non-rotatable, and preferably, easilyexchangeable. This very effectively takes into account that thestructural elements which prevent the synthetic plastic material engagedby the screw from merely turning along with it, that is the ribs, aresubject to extreme wear when processing synthetic plastic materialcontaining abrasive contaminants, especially waste material. Such aseparate sleeve is a cost-effective solution to this problem, especiallyif it is easily exchangeable. Otherwise, if the ribs confining thepockets or the grooves became worn to a degree no longer deemedacceptable, the entire feed section of the screw housing would have tobe replaced at great expense. Although a separate sleeve comprising thegrooves or the ribs confining the grooves is also subject to extremewear, however this sleeve can be manufactured relativelycost-effectively due to its simple geometric features and thusconstitutes a wearing part which is exchanged as required. Owing to itssimplicity of manufacture, the sleeve can also be made of awear-resistant material, especially of hard metal.

In accordance with a further variant of the invention, provision is madein the area of the inlet opening for only one single pocket-likeenlargement, preferably with a constant depth, which extends around theentire circumference of the screw with the exception of the inletopening and which then continues in the form of several grooves. Testshave shown, namely, that the conveying efficiency of the screw can besufficiently regulated if only a single, adjustable outermost rib isplaced directly next to the inlet opening. This arrangement furthersimplifies the manufacture of the screw housing section with the inletopening.

In operation, the synthetic plastic material engaged by the screw ispressed in gradually between the screw flights due to the decreasingdepth of the grooves. Preferably, the arrangement here should be suchthat the depth of the grooves continuously decreases in the direction oftransport of the screw in order to prevent blockages of the material,and preferably, the bottom of at least one groove is shaped like thesurface of the envelope of a cone.

In the scope of the invention, it is recommended that the length of thegrooves as measured in the axial direction of the screw be dimensionedshorter than the width of inlet opening as measured in the samedirection, and, preferably, the groove length measured in this wayshould be 0.5 to 3 times the screw diameter. The shorter the grooves,especially in a separate housing section, the simpler it will be tomanufacture this component.

BRIEF DESCRIPTION OF THE FIGURES

In the drawing examplative embodiments of the subject matter of theinvention are schematically shown.

FIG. 1 shows a first embodiment in a vertical section taken along theline I--I of FIG. 2.

FIG. 2 illustrates a horizontal section to FIG. 1 at the height of thescrew axis.

FIG. 3 illustrates a further embodiment in a vertical section takenperpendicularly to the screw axis and

FIG. 4 is a section taken along Line IV--IV of FIG. 3.

FIG. 5 shows in a larger scale a somewhat modified form of execution ofan exchangeable sleeve with grooves.

DETAILED DESCRIPTION OF THE INVENTION

In the embodiment shown in FIGS. 1 and 2, a receptacle 1 is provided forto hold the synthetic plastic material to be processed, which syntheticplastic material generally consists of thermoplastic waste material ofall kinds. The receptacle has a bottom 2 and side walls 3. The syntheticplastic material is fed into the receptacle 1 from the top through afeeding device not shown, if need be, after being pretreated, e.g.cleaned or freed of coarse contaminants, e.g. stones or metal parts. Inthe receptacle 1, the synthetic plastic material is treated by at leastone rotating tool 4, which in this form of execution has sharp operatingedges 5 which act on the synthetic plastic material and are preferablyformed by bent blades, so that the synthetic plastic material iscomminuted and/or mixed during the rotation of tool 4. The syntheticplastic material is reduced in size if it is supplied in form of largerelements (sheets, bottles, etc.) and it is mixed if it is alreadyprocessed into particle form (e.g. granules). In most cases, thereby thesynthetic plastic material is also heated, making it easier toplasticize later on. The tools 4 are mounted on a rotor disk 6, arrangedjust above the bottom 2 and driven for rotational movement in thedirection of the arrow 9 (FIG. 2) by a motor 8 via a shaft 7, whichintersects the bottom 2.

In the side wall 3, the receptacle 1 has a discharge opening 10, whichis connected--if need be by means of a short tube--to an inlet opening11 of the housing 12 of a screw 13. The screw is driven by a drivemeans, not shown, in the direction of the arrow 14 to rotate around itslongitudinal axis 15 and conveys the synthetic plastic material engagedby it in the direction of the arrow 16. If the screw 13 is not merely aconveying screw, but a screw which conveys the material to an extruderhead, the synthetic plastic material is plasticized. The volutions ofthe screw 13 extend beyond the inlet opening 11, when seen opposite thedirection 16 of transport, and give way there to a thread 18 which actsas a seal, the pitch of which thread has the same direction as that ofthe screw flights 17 and which, for the sake of expediency, has narrowerflights. The screw 13 or its housing 12 can be fitted with a heaterand/or with a cooler in order to be able to keep the synthetic plasticmaterial being conveyed at the desired temperature.

The substantially rectangular inlet opening 11, which is located on theside wall of the housing 12 of the screw 13, is dimensioned to berelatively large; it extends in the direction of the axis of the screw13 over about two flights 17 in the embodiment shown here and, in anycase, over at least half of a flight. The height of the inlet opening11, as measured in the direction of the axis of the receptacle 1, isabout as large as the diameter of the screw flights 17. This ensuresthat the screw 13 can take in the incoming synthetic plastic materialfrom receptacle 1 in large quantities. The housing 12 of the screw 13 isattached substantially tangentially to the side wall 3 of the receptacle1 such that--as seen in the direction of the screw axis 15--thecircumference of the screw 13 is somewhat further away from the centerof the receptacle 1 than the interior surface of the wall 3 (FIG. 1) ormore or less flush with the latter. The wall of the housing 12 continuesin the form of a horizontal section to the bottom edge 19 of the inletopening 11, where it connects with a curved wall section 20 which leadsto the bottom 2 of the receptacle 1. The bottom edge 19 of the inletopening 11 is at about the same height as the upper surface of the rotordisk 6. The synthetic plastic material is therefore pressed by a feederdevice 21 from the side between the flights 17 of the screw 13. This wayof introducing the synthetic plastic material into the screw housing 12guarantees an especially favorable overall design, a point which will beelaborated on below.

The rotating tools 4 in the receptacle 1 constitute a feeder device 21which presses the synthetic plastic material it processes from thedischarge opening 10 into the inlet opening 11, doing so, on the onehand, by the exertion of centrifugal force on the rotating syntheticplastic material and on the other, by the blade-type action of the tools4. As a result of this, a stuffing force is exerted on the syntheticplastic material such that the synthetic plastic material subsequentlypressed in, in turn, presses the synthetic plastic material in front ofit into the inlet opening 11, this pressure being exerted largely in adirection having a substantial component in direction of the axis of theinlet opening 11, that is perpendicular to the direction of the axis 15of the screw 13. This, on the one hand, counteracts bridging of thesynthetic plastic material in the area of the inlet opening 11, and onthe other hand always a sufficient amount of synthetic plastic materialis pressed between the individual screw flights 17 so that always anample supply of synthetic plastic material is supplied to the screw 13,even if this material is only in very loose form, e.g. in form offlakes, foils, chips, etc. To further improve the screw's ability toengage the synthetic plastic material entering the housing 12, a freespace is provided for in screw housing 12 in addition to the screwflights 17, which space is formed by a pocket-like enlargement 22 of thescrew housing 12. This enlargement 22 takes the form of a hollowcylinder over large sections of the circumference of the screw housing12 and extends in the direction of the axis of the screw 13 over atleast one screw flight 17, in the embodiment shown over approx. 2.5screw flights (FIG. 2). The enlargement 22 is directly connected to theinlet opening 11. The width of the enlargement 22, as measured in thedirection of the circumference of the screw 13, is preferably constantalong its entire length so that the pocket formed by the enlargement 22has essentially the same depth throughout. The enlargement 22 directlyadjoins the sealing thread 18 and extends beyond both sides of the inletopening 11 in the direction of the axis of the screw 13. The depth ofthe enlargement 22, as measured radially from the screw 13, is about 5to 10% of the diameter of the screw 13. At the bottom edge 19 of theinlet opening 11, the enlargement 22 gives way steplessly to the inletopening 11 (FIG. 1), forming at this point essentially a horizontalcontinuation of the upper surface of the rotor disk 6. By contrast, atthe upper edge of the inlet opening 11, that is the edge of the inletopening 11 at which the screw 13 rotates in direction towards thereceptacle 1 (arrow 14), the enlargement has an adjustable wall piece 23which consists of a rib 24 which is guided in a guiding slot 25 of thescrew housing 12 and can be continuously adjusted by means of adjustingdevices 26, e.g. adjusting screws, to be closer to the screw 13 orfurther away from it. For the sake of expediency, the adjustment rangecovers the entire depth of the pocket 22. The front face of the rib 24opposite the screw 13 is, for the sake of expediency, shaped tocorrespond with the circumference of the enlargement 22. In itscompletely retracted position, the rib 24 therefore constitutes a smoothcontinuation of the exterior wall of the pocket-like enlargement 22 sothat the rib 24 creates no resistance to the synthetic plastic materialengaged by the screw 13. By contrast, when in its front-most positiontoward the screw 13, the rib 24 contacts with its front face thecircumference of the flights 17 of the screw 13 and therefore exertsconsiderable resistance against the synthetic plastic material engagedby the screw 13. In this latter position, the rib 24 closes off thepocket-like enlargement 22 at least partially from the interior of thereceptacle 1 so that the synthetic plastic material in the pocket-likeenlargement 22 is continuously pressed between the flights 17 of thescrew 13 and therefore cannot merely rotate along with the screw 13 butrather is transported in the direction of the axis of the screw. Bycontrast, when the rib 24 is in its retracted position, a part of thesynthetic plastic material engaged by the screw 13 in the inlet opening11 is conveyed back into the inlet opening 11, thus reducing theconveyance performance of the screw 13 in the direction of the arrow 16.If intermediate positions of the rib 24 are selected, the conveyingefficiency of the screw 13 can be set as desired between maximum andminimum and therefore adjusted to meet prevailing conditions. This isespecially of advantage when the nature of the synthetic plasticmaterial is variable, e.g. of varying density, since it preventsoverloads of the screw 13 and its drive means. Directly adjoining thepocket-like enlargement 22 there is an area 27 of the housing 12 whichhas several grooves 28 on its interior surface, which grooves 28 areseparated from each other by rigid and unmoving ribs 29 projecting fromthe periphery wall of this area 27. The grooves 28 and the ribs 29 runin the direction 16 of the screw axis 15 in the embodiment shown, butthey can also be wound helically around this axis 15, however, they mustalways have a substantial component of their longitudinal direction,which is directed in the direction of the screw axis 15. The ribs 29 areof such a height that their surfaces facing the screw 13 rest on theperiphery of the screw flights 17 and therefore practically form asupport for the screw 13 in the area 27. The depth of the grooves 28decreases the further away from the pocket-like enlargement 22 they are,preferably continuously and steplessly. The arrangement in theembodiment shown is such that the depth of the grooves 28 at the end atwhich the conveyed synthetic plastic material enters is the same depthas that of the enlargement 22 so that no step is created at thetransition from the enlargement 22 to the grooves 28, however, each rib29 does form such a step. On the other front face of the area 27, thereis a smooth transition between the grooves 28 and the outer diameter ofthe screw, that is the bottom of the grooves 28 reaches the top of theribs 29. For this reason, on this front face 30 (FIG. 2), the grooves 28and the ribs 29 give way, steplessly for the sake of expediency, to thecylindrical wall 31 of the screw housing 12.

Suitably, the side walls of the grooves 28 extend such that thesynthetic plastic material being transported by the screw 13 catches onthem. Preferably, to this end, that side wall of each successive groove28--as seen in the direction of rotation of the screw 13 (arrow14)--should be inclined opposite the direction of rotation of the screw.The ribs 29 between the grooves 28 therefore have a more or lesstrapezoidal cross section. The grooves 28 need not all be equal inwidth, although this would simplify the manufacturing process. The sameis true of the ribs 29. For the sake of expediency, the ratio of thewidth of the grooves 28 to the width of the ribs 29, as seen in thedirection of rotation of the screw 13 (arrow 14), should be 0.5:1 to2:1. In the embodiment shown (FIG. 1), this ratio is about 1:1. Thebottom 32 of each groove 28 can be shaped like the envelope of a conebut a flat bottom 32 is also possible.

To simplify manufacturing, the grooves 28 and the ribs 29 aresubstantially shorter than the length of the enlargement 22 as measuredin the direction of the axis of the screw 13 and also shorter than theextent of the inlet opening 11 as measured in the same direction.Suitably, the ratios between the above lengths of the enlargement 22 tothat of the grooves 28 as measured in the indicated direction should be3:1 to 1.5:1; for the sake of expediency, about 2:1. It is particularlyadvantageous here if a provision can be made to have the grooves 28 bein a separate section of the screw housing 12, which section is executedas an easily exchangeable sleeve 32 (FIG. 2). This sleeve 33 is insertedinto a cylindrical recess 34 of the screw housing 12 and is attachedthere so that it cannot turn, e.g. by means of a wedge, not shown, or asimilar device. To ensure the easy replacement of this sleeve 33, whichis a wearing part and which, for the sake of expediency, is made ofwear-resistant material, especially hard metal, the sleeve 33 is locatedbetween two sections 35 and 36 of the screw housing 12 which are screwedtogether at flanges 37. The aforementioned precaution against the sleeve33 turning can also be undertaken by putting projections on the frontface on the downstream side, e.g. toothing, claws, or the like, whichprojections then engage in corresponding recesses of the screw housingsection 35. The relatively short axial length of the sleeve 33 makes iteasy to manufacture so that this part subject to such heavy loading canbe exchanged very cost-efficiently.

The edge of the inlet opening 11 on the downstream side is executed asan inclined surface 38 (FIG. 2), preferably conical, and there is asmooth transition between it and the front face of the ribs 29 or thegrooves 28, respectively. As FIG. 2 indicates, the front face 39 of theribs 29 on the upstream side can be inclined, preferably conical,whereby this inclination can be flush with the inclined surface 38 ofthe inlet opening 11. This enhances the catching of synthetic plasticmaterial in the pocket-like enlargement 22 for the purpose of beingconveyed by the screw 13 in the direction of the arrow 16.

In operation, the rotational movement of the tools 4 causes thesynthetic plastic material to be pressed by centrifugal force, supportedby the blade-like action of the angle-bent tools 4, into the dischargeopening 10 of the receptacle 1 and through this opening into the inletopening 11 of the screw housing 12. The synthetic plastic material beingstuffed in in this way enters between the individual flights 17 of thescrew 13 and into the pocket-like enlargement 22 and fills in thesespaces. Through the turning of the screw 13, the synthetic plasticmaterial is carried along and pressed into the grooves 28 of the sleeve33. The rib 24 assists this pressing of the synthetic plastic materialinto the grooves 28 to varying degrees depending on how far the rib 24projects upward from the bottom of the enlargement 22. Due to thedecreasing depth of the grooves 28 and the thrust exerted by thesynthetic plastic material subsequently being fed in, the syntheticplastic material is by and by pressed between the screw flights 17 ofthe screw 13 until it is completely between the screw flights 17 in thesection 35 of the screw housing 12. Due to this pressing in of syntheticplastic material, the screw 13 achieves a high degree of conveyingefficiency which can bring it to the limits of its loadabilty. If thislimit is close to being reached, the conveying efficiency of the screw13 can be reduced by retracting the rib 24 since this causes part of thesynthetic plastic material carried along by the screw in the enlargement22 in the direction of arrow 14 to be conveyed back into the inletopening 11. To be especially effective, the length of the rib 24, asmeasured in the direction of the axis of the screw 13, extends at leastover the width of the inlet opening 11, as measured in the samedirection (FIG. 2).

In order to better absorb the stuffing pressure exerted by the syntheticplastic material on the screw 13 and also the bending load exerted bysolids on the screw 13, at least one additional longitudinal rib can beprovided in the pocket-like enlargement 22, which rib acts as a supportfor the screw 13 and, if need be, can be adjustable, as with the rib 24.However, it is preferable to have only one single pocket-likeenlargement 22 since one is usually enough and having just onesimplifies the manufacture of the feed section of the screw housing 12.

The adjustable wall piece 23 need not necessarily be executed as a rib24, because a wall piece 23 can also be provided for, which can beswiveled in relation to the rest of the wall of the pocket-likeenlargement 22 such that the pivotal point is on the feed side. In thisway, the synthetic plastic material carried along by the screw 13 can bemore gradually pressed into the screw flights and not so suddenly as isthe case with the rib 24. In terms of design, however, the rib 24 ismuch simpler and is therefore the preferred embodiment. 10 Theembodiment variant shown in FIGS. 3 to 5 differs from that shown inFIGS. 1 and 2 substantially only by the way in which the syntheticplastic material is supplied into the inlet opening 11 of the screwhousing 12. For this purpose, a hopper 40 is connected to the upwardfacing inlet opening 11 of the screw housing 12, which hopperconstitutes the receptacle 1 for the synthetic plastic material, andwhich hopper contains a screw 41 as a feed unit 21, which rotates, thusexerting the necessary stuffing action on the synthetic plastic materiallocated in the hopper 40. In this embodiment the width of the inletopening 11, as measured in the direction of the axis of the screw 13, isdimensioned just slightly larger than the screw flight height and thelength of the sleeve 33, as measured in the same direction, isapproximately the same size as the width of the inlet opening 11. Asillustrated in FIG. 5, the sleeve 33 can also be executed to be shorter,thus increasing the steepness of the groove bottom 32 formed in theshape of the envelope of a cone.

The tangential connection of the screw housing 12 to the receptacle 1makes it possible to arrange the drive means for the screw 13 on one endof the latter and an extruder head, or the like, on the other end sothat the synthetic plastic material can be extruded by the screw 13without the need for a deviation. This ensures an especially high degreeof efficiency.

We claim:
 1. An apparatus for reprocessing thermoplastic syntheticplastic material, comprising a receptacle for the synthetic plasticmaterial to be processed, said receptacle having at least one rotatingtool which conveys the synthetic plastic material through a dischargeopening of the receptacle such that a component of a stuffing force isexerted on the synthetic plastic material in the axial direction of thisdischarging opening and transmitted thereto always by synthetic plasticmaterial subsequently being pressed in, said discharge opening leadingto an inlet opening for the synthetic plastic material arranged in aside wall of a screw housing of a screw, whereby the interior surface ofsaid screw housing of the screw in the area of the inlet opening has apocket-like enlargement, which provides an additional space toaccommodate synthetic plastic material and which is confined by anadjustable wall piece on that edge of the inlet opening on which thedirection of rotation of the screw is directed towards the receptaclesaid wall piece allowing the enlargement to be at least partially closedoff from the receptacle whereby the enlargement gives way to severalgrooves at least in the area of the screw housing adjoining it in thetransport direction of the screw and whereby said grooves are of a depthwhich decreases to the screw diameter in this area and in which thegrooves are separated from each other in this area by rigid ribsprojecting toward the screw relative to the wall of the enlargement ofthe screw housing, each of these grooves providing a space for thesynthetic plastic material, the longitudinal direction of which spacehaving a substantial component extending in the axial direction of thescrew.
 2. Apparatus according to claim 1, wherein the ribs extend inradial direction of the screw housing to the circumference of the screwflights.
 3. Apparatus according to claim 1 wherein the grooves areprovided only in an area of the screw housing adjoining the inletopening.
 4. Apparatus according to claim 1, wherein the area of theinlet opening only one single pocket-like enlargement is provided, whichextends around the entire circumference of the screw with the exceptionof the inlet opening and which continues in form of several grooves. 5.The apparatus according to claim 4 wherein the one single pocket-likeenlargement has a constant depth.
 6. Apparatus according to claim 1,wherein the depth of the grooves continuously decreases in direction oftransport of the screw.
 7. The apparatus according to claim 6 whereinthe bottom of at least one groove is shaped like the surface of theenvelope of a cone.
 8. Apparatus according to claim 1, wherein thelength of the grooves as measured in the axial direction of the screw isshorter than the width of the inlet opening as measured in the samedirection.
 9. The apparatus according to claim 8 wherein the length ofthe groves is 0.5 to 3 times the screw diameter.
 10. Apparatus accordingto claim 1, wherein the grooves have the same width, as measured in thedirection of the circumference of the screw.
 11. Apparatus according toclaim 1, wherein the ribs have the same width, as measured in thedirection of the circumference of the screw.
 12. Apparatus according toclaim 1, wherein the ratio of the width of the grooves to the width ofthe ribs, as measured in the direction of the circumference of thescrew, is 0.5:1 to 2:1.
 13. The apparatus according to claim 12 whereinsaid ratio is about 1:1.
 14. Apparatus according to claim 1, wherein theribs and the grooves run in the longitudinal direction of the screw. 15.Apparatus according to claim 1, wherein the grooves are provided in aseparate section of the screw housing, which section is executed as asleeve installed in the remainder of the screw housing such that itcannot turn.
 16. Apparatus according to claim 15, wherein the sleeve ismade of wear-resistant material.
 17. The apparatus according to claim 15wherein the sleeve is detachable from the screw housing such that it iseasy to exchange.
 18. Apparatus according to claim 1, wherein theenlargement has a constant depth in the direction of the circumferenceof the screw.
 19. Apparatus according to claim 1, wherein the edge ofthe inlet opening is executed as an inclined surface which gives way tothe area of the screw housing adjoining the enlargement.
 20. Theapparatus according to claim 19 wherein the inclided surace is conical.21. Apparatus according to claim 1, wherein the adjustable wall piece isformed by a rib, the adjustment range of which is as large as the depthof the pocket-like enlargement.